The subject of the invention is an impact modified polyamide moulding compound which contains a partially aromatic copolyamide and a polyolefinic impact modifier and which on the basis of the selection of the individual components has high impact strength and elongation at break and high thermal aging resistance. The moulded articles produced therefrom are also a subject of the invention.
Conventionally known polyamides such as PA6 and PA66 are easy to process and have high melting points and high thermal dimensional stability, especially if they are reinforced with glass fibres or contain mineral fillers. However, they typically have high water absorption of up to 10% on storage in water. For many applications with high requirements for dimensional stability even under moist conditions these aliphatic polyamides cannot be used. Furthermore, with the absorption of water, the mechanical properties also change; the rigidity and the strength are reduced severalfold by water absorption.
Partially aromatic polyamides have reduced water absorption compared to PA6 and PA66, and the mechanical properties are largely maintained after water absorption. In addition, they have melting points which sometimes lie markedly above that of PA66 and would therefore in principle be suitable for applications with prolonged exposure to high temperatures, for example in the engine compartment of an automobile. Since PA6T has such a high melting point that it is no longer processable, in practice copolyamides are used, for example PA6T/6I, PA6T/66 or PA6T/6, the melting point whereof has been lowered sufficiently for the polymers to be processable. With suitable composition, such copolymers are highly crystalline, have melting points in the range around about 300° C. and high rigidity; however, they are generally very brittle and have very low elongation at break.
With polyamide moulding compounds, it is generally customary to improve the toughness and elongation at break by admixture of an impact modifier. In most cases, a polyolefin rubber which has been grafted with an unsaturated dicarboxylic acid anhydride such as maleic anhydride in order to effect phase bonding of the dispersively distributed rubber to the polyamide matrix is used as the impact modifier. In particular, an ethylene-propylene rubber which contains 0.5 wt. % of grafted maleic anhydride may be used.
With moulding compounds based on partially aromatic polyamides also, efforts were made in the past to improve their toughness and elongation at break by use of an impact modifier.
EP 1 988 113 A1 describes a polyamide moulding compound based on a copolyamide 10T/6T, which is formed from the monomers 1,10-decanediamine, 1,6-hexamethylenediamine and terephthalic acid. Copolyamides have relatively high melting points in the range around 300° C.; the processing window is therefore relatively small. A range of impact modifiers which can be added are stated, with priority being given to ethylene-propylene rubber; however, differences are not discussed in detail and the retention of the mechanical properties after thermal aging is not a central theme. Our own experiments have shown that with such impact modified moulding compounds the elongation at break is low.
EP 2 325 260 A1 describes a polyamide moulding compound based on a copolyamide which is made up of 1,10-decanediamine, 1,6-hexamethylenediamine, terephthalic acid and at least one further polyamide-forming monomer. Here also, a range of impact modifiers which can be added are named, with priority being given to ethylene-propylene rubber. Here also, our own experiments have shown that with such impact modified moulding compounds the elongation at break is low.
JP 04-202560A describes that after exposure to heat, moulding compounds of a hexamethylenediamine-isophthalic acid-terephthalic acid copolymer and maleic anhydride-functionalized ethylene-1-butene rubber do not differ in their toughness and elongation at break from moulding compounds which contain other rubbers.
WO 2005/018891 and in JP 2010-202724A describe moulding compounds which contain an aromatic polyamide, a functionalized impact modifier and a copper stabilizer. Antioxidants can also be added. The moulding compounds are used for multilayer pipes.
However, the moulding compounds known from this state of the art have a number of disadvantages, in particular inadequate thermal aging resistance. Furthermore, their processing properties are poor. Their mechanical properties, in particular the impact resistance and elongation at break are so poor that their use in moulded articles such as for example pipes in the engine compartment of an automobile or under similar conditions is not advisable.
Easily processable moulding compounds are described in WO2012/106309; they contain a partially aromatic polyamide with a mainly aliphatic content and an aliphatic polyamide and a functionalized rubber. However, their thermal dimensional stability is comparable with fully aliphatic polyamides and far from adequate for demanding applications. Similar materials are described in US2013/0115401A1.
The object of the present invention was to avoid these disadvantages and in particular to provide readily processable, high melting, impact resistant moulding compounds with high elongation at break based on a partially aromatic polyamide, which essentially retain their advantageous mechanical properties even after prolonged use at high temperatures.